1. Technical Field
The present invention relates to a vehicle speed determination system for a vehicle which banks while curving, a stability control system for such a vehicle, and a saddled vehicle incorporating the same.
2. Description of the Related Art
In attitude control techniques (vehicle stability control techniques) for motorcycles, such as a traction control system (TCS) and an anti-lock braking system (ABS), it is commonplace to utilize a “slip amount” or “slip rate” of wheels which is obtained from a difference in wheel speed between the front wheel and the rear wheel.
A slip rate λ is typically expressed by the following equations.λ=(V−Vr)/V  during brakingλ=(V−Vr)/Vr  during driving
A slip amount can be expressed as (V−Vr), for example.
Herein, V is a vehicle speed, and Vr is a wheel speed of the rear wheel (driving wheel). Generally speaking, a “vehicle speed” is a moving velocity of a vehicle with respect to the road surface. A “wheel speed” is a velocity in a tangential direction on the outer peripheral surface of a wheel, as referenced to the rotation axis of the wheel. A wheel speed is proportional to “the rotation speed (revolutions per unit time) of the wheel” and “the rotating radius of the wheel”, and generally expressed as a product of the “rotational angular velocity of the wheel” and “the rotating radius of the wheel”. The unit of wheel speed may be “km/hour”, for example, although not being limited thereto. According to the above equations, when the vehicle speed V is equal to the wheel speed Vr of the rear wheel, the slip rate λ and the slip amount will equal zero.
As an attitude control technique (vehicle stability control technique) for motorcycles, Japanese Laid-Open Patent Publication No. 2008-126988 (hereinafter “Patent Document 1”) discloses a brake fluid pressure controlling apparatus for motorcycles. This controlling apparatus includes wheel speed sensors for detecting the respective wheel speeds of the front and rear wheels, and controls a braking force adjustor based on the wheel speed detected by each wheel speed sensor.
In attitude control techniques (vehicle stability control techniques) for motorcycles, conventionally, vehicle speed is acquired by detecting the wheel speed of the front wheel, which is not a driving wheel. Specifically, given a wheel speed Vf of the front wheel, it is assumed that V=Vf; then, a measured value of the wheel speed Vf of the front wheel is substituted for the vehicle speed V in the aforementioned equations for calculating a slip rate or a slip amount.
Moreover, the controlling apparatus of Patent Document 1 includes wheel speed sensors for detecting the respective wheel speeds of the front and rear wheels, and controls a braking force adjustor based on the wheel speed detected by each wheel speed sensor. In this controlling apparatus, a vehicle speed is estimated from a front-wheel wheel speed which is detected by the wheel speed sensor for the front wheel. The brake fluid pressures of the braking devices are adjusted in accordance with a speed difference between this estimated vehicle speed and a rear-wheel wheel speed which is detected by the wheel speed sensor for the rear wheel.
Thus, under the conventional techniques, the vehicle speed V is a measured wheel speed value of the front wheel, or an estimated value based on the wheel speed of the front wheel. The aforementioned methods of determining the vehicle speed V are both based on the premise that the vehicle body is traveling straight. However, a saddled vehicle is capable of banking while curving.